1. Field of the Invention
This invention relates to circuit breakers and, more particularly, to circuit breakers wherein electromagnetic repulsion forces assist in current limitation.
2. Description of Related Art
The interrupting rating of a circuit breaker can be economically increased by using a current limiting set of contacts which may be added onto an existing circuit breaker or integrally formed in a breaker. The current limiting contacts limit the amount of current that would flow through the breaker under short circuit conditions which enables the breaker to be designed to interrupt that maximum amount rather than an unlimited amount from a source capable of generating extremely high short circuit currents. Typically, current limiting circuit breakers operate on an electromagnetic repulsion principle wherein as the magnitude of the fault current increases to a pre-selected magnitude, the magnetic forces build which tend to repel a movable contact away from a stationary contact thereby interrupting the circuit. In the design of current limiting contacts, it is important that when the contacts are closed, there is sufficient contact pressure to assure minimal resistance to current flow but that the means for insuring the requisite contact pressure does not inhibit the fast operation under fault conditions.
The actual current limiting apparatus of prior art circuit breakers generally comprise spaced conductors and associated structure which is capable of causing a repulsive force to arise between the conductors. Examples of prior art circuit breakers having these elements are disclosed in U.S. Pat. No. 4,001,738 to Terracol et al. and U.S. Pat. No. 3,887,888 to Bayles et al. Terracol et al. describe a circuit breaker utilizing an electromagnetic repulsive force to effectuate an abrupt separation of a movable contact from a stationary contact. In Terracol et al.'s breaker, current flows in one direction through the stationary contact arm and in second direction through the movable contact arm. A conducting induction plate containing a return conductor forms a cage imprisoning the movable contact arm. The secondary currents in this induction plate tend to force the movable contact arm vigorously away from the stationary contact arm. The patent to Bayles et al. depicts a high current switch provided with a U-shaped stationary yoke piece provided around stationary and movable contacts.
While prior art circuit breakers, such as those identified above, generally operate acceptably in response to high level fault conditions, those skilled in the circuit breaker manufacturing art recognize or should appreciate the undue complexity and high cost of manufacturing the prior art circuit breakers. With regard to the Bayles et al. breaker, for example, it is necessary to provide proper positioning of components and to provide an insulating barrier between the yoke and movable conductor during manufacture. The technique most often employed to accomplish this relies upon a complex molded plastic housing into which the yoke is inserted, which in turn must be secured and suitably positioned around the conducting members. Various supplementary insulating materials (fiber sheet, silicone adhesive, and so on) are still often required to maintain adequate dielectric strength between the current limiting apparatus and other breaker components. Overall complexity is further increased by, in many cases, having separate fasteners to secure both the stationary conducting member, as well as the current limiting apparatus, to the breaker housing.
Accordingly, it should be appreciated that there is a need for a simple current limiting assembly for circuit breakers that can be easily and inexpensively manufactured.